Introduction

The breeding system of a species has important ecological and evolutionary consequences. Among the mammals, differences between the sexes in body size and the intensity of secondary sexual characters such as coat color or ornamentation (antlers, for instance) in males are significantly correlated with reproductive strategies. Charles Darwin (1871, Vol. II) noted that, among monogamous seals, the sexes were approximately equal in body size, while in polygynous seals, the males were up to six times larger than the females. The males of polygynous species are also more likely to sport elaborate ornamentation, such as large antlers or horns, which in Darwin's words are "singularly ill-fitted for fighting" (p. 251) but are "used chiefly or exclusively for pushing and fencing" (p. 253).

The elephant is a polygynous mammal, as are 95% of all mammals. Polygyny has been defined by William Shields as a situation when "more females than males breed, with the result that variance in reproductive success is greater in males than in females. The greater the difference in variance, the greater the degree of polygyny." The degree of dimorphism between the sexes is a good indicator of the degree of polygyny in a species. The elephant is one of the most sexually dimorphic of mammals. In African elephants, a full-grown male weighs twice as much as a full-grown female, making these among the most sexually dimorphic (and also polygynous) of all mammals.

One demographic consequence of polygyny is that sex ratios, especially of adults, are biased toward females because the males suffer higher death rates. There could be several reasons for this asymmetry in survivorship. Males pay an additional metabolic price in the process of attaining a larger body size, making them more susceptible to nutritional stress and diseases. There are higher costs (either injuries or death) associated with male-male competition or risks of emigrating from the natal family. Elephant populations universally show a preponderance of females over males, with the disparity progressively increasing with age class. Spatial segregation between the sexes on a seasonal basis may also be a feature of polygynous species. Males and females may use different areas because of variation in nutritional demands or as part of their antipreda-tory strategies. While there is no evidence for exclusive cow (i.e., female-led family) areas, there is evidence for exclusive bull areas among elephants.

Whatever the social system of a species, the sexes have to meet, even if only during a certain time of the year, for breeding. There are two determinants of mating: dominance hierarchies based on the outcomes of male contests and female choice of mates with certain desirable qualities. For a long time, biologists believed that mating success was decided entirely by the competition among males. Even though Charles Darwin had cautiously proposed that females of some species might select their mates, it took over a century for this idea to be tested and confirmed empirically.

Reproduction in elephants has been described, often in rather imaginary terms, since ancient times. From the ancient Sanskrit and Tamil literature of India, to the writings of the Greek philosopher Aristotle, to the zoological literature of the eighteenth and nineteenth centuries, we obtain glimpses of the mating game in the largest of terrestrial mammals. Scientific details of the reproductive biology of elephants, however, emerged from three major lines of investigation that began around the mid-twentieth century. The elephant control programs in eastern Africa at that time and later in southern Africa provided the material for detailed investigations of the anatomy and histology of reproductive organs. Seminal papers in 1953 and 1964 by J. S. Perry of reproductive biology in African elephants provided the framework for later descriptions of reproductive structures and function. Since then, several physiological studies, chiefly of hormonal control of reproduction, of both Asian and African elephants in captivity have revealed a rather complex female estrous cycle. At the same time, behavioral studies of mating from captive and wild situations have added substantially to our knowledge of reproductive strategies of elephants.

In this chapter, I first describe the estrous cycle of a female elephant and how this state is conveyed to the males. I then describe the phenomenon of musth, a rutlike condition in bull elephants. A bull in musth undergoes important physiological changes and exhibits a range of specific behaviors designed to increase its mating success. After describing the behavioral and physiological correlates of musth from studies of both Asian and African elephants, I analyze the evolutionary significance of this phenomenon in the framework of game theory as applied to competing males and female choice of mates. Finally, I consider a possible role for tusks as an indicator of genetic fitness of male elephants.

0 0

Post a comment